I have used tripit for a while. I don't travel that much, but when I do tripit makes it very easy to get an consistent itinerary for your trips. For example, if you make reservations for air and hotel at separate times, all you have to do is forward each individual itinerary to tripit and it will generate a "trip" with all of your information. In addition to that, tripit makes it very easy to share your itinerary.
Normally, you just use their web interface to access their site, and they even have a mobile version that looks good on the iPhone. But that web site doesn't help you if you don't have network access.
I just saw that they have an iPhone application that syncs with their site to get the list of current or upcoming trips. I downloaded it and it works great. I can access my itinerary even with out network access (for example now I am flying somewhere above Kansas.). This is great if you need to know when your connecting flight departs.
I also like the fact that they use OAuth for authentication. I could easily revoke access to this app if I ever lost my phone.